Cyclohexylbenzene can be produced from benzene by the process of hydroalkylation or reductive alkylation. In this process, benzene is heated with hydrogen in the presence of a catalyst such that the benzene undergoes partial hydrogenation to produce a reaction intermediate such as cyclohexene which then alkylates the benzene starting material. Thus, U.S. Pat. Nos. 4,094,918 and 4,177,165 disclose hydroalkylation of aromatic hydrocarbons over catalysts which comprise nickel- and rare earth-treated zeolites and a palladium promoter. Similarly, U.S. Pat. Nos. 4,122,125 and 4,206,082 disclose the use of ruthenium and nickel compounds supported on rare earth-treated zeolites as aromatic hydroalkylation catalysts. The zeolites employed in these prior art processes are zeolites X and Y. In addition, U.S. Pat. No. 5,053,571 proposes the use of ruthenium and nickel supported on zeolite beta as the aromatic hydroalkylation catalyst. However, these earlier proposals for the hydroalkylation of benzene suffered from the problems that the selectivity to cyclohexylbenzene was low, particularly at economically viable benzene conversion rates, and that large quantities of unwanted by-products, particularly cyclohexane and methylcyclopentane, were produced.
More recently, U.S. Pat. No. 6,037,513 has disclosed that cyclohexylbenzene selectivity in the hydroalkylation of benzene can be improved by contacting the benzene and hydrogen with a bifunctional catalyst comprising at least one hydrogenation metal and a molecular sieve of the MCM-22 type. The hydrogenation metal is preferably selected from palladium, ruthenium, nickel, cobalt, and mixtures thereof, and the contacting step is conducted at a temperature of 50° C. to 350° C., a pressure of 100 kPa to 7000 kPa, a benzene to hydrogen molar ratio of 0.01 to 100 and a weight hourly space velocity (WHSV) of 0.01 hr−1 to 100 hr−1. The '513 patent discloses that the resultant cyclohexylbenzene can then be oxidized to the corresponding hydroperoxide and the peroxide decomposed to the desired phenol and cyclohexanone.
One disadvantage of this process is that it produces impurities such as cyclohexane and methylcyclopentane. These impurities represent loss of valuable benzene feed. Moreover, unless removed, these impurities will tend to build up in the system, thereby displacing benzene and increasing the production of undesirable by-products. Thus, a significant problem facing the commercial application of cyclohexylbenzene as a phenol precursor is removing the cyclohexane and methylcyclopentane impurities.
One solution to this problem is proposed in U.S. Pat. No. 7,579,511 which describes a process for making cyclohexylbenzene in which benzene undergoes hydroalkylation in the presence of a first catalyst to form a first effluent composition containing cyclohexylbenzene, cyclohexane, methylcyclopentane, and unreacted benzene. The first effluent composition is then separated into a cyclohexane/methylcyclopentane-rich composition, a benzene-rich composition, and a cyclohexylbenzene-rich composition and the cyclohexane/methylcyclopentane-rich composition is contacted with a second, low acidity, dehydrogenation catalyst to convert at least a portion of the cyclohexane to benzene and at least a portion of the methylcyclopentane to linear and/or branched paraffins and form a second effluent composition. The benzene-rich composition and the second effluent composition can then be recycled to the hydroalkylation step. However, one problem with this process is that cyclohexane and methylcyclopentane have similar boiling points to that of benzene so that their separation by conventional distillation is difficult.
Another solution is proposed in International Patent Publication No. WO2009/131769, in which benzene undergoes hydroalkylation in the presence of a first catalyst to produce a first effluent composition containing cyclohexylbenzene, cyclohexane, and unreacted benzene. The first effluent composition is then divided into a cyclohexylbenzene-rich composition and a C6 product composition comprising cyclohexane and benzene. At least part of the C6 product composition is then contacted with a second catalyst under dehydrogenation conditions to convert at least part of the cyclohexane to benzene and produce a second effluent composition which comprises benzene and hydrogen and which can be recycled to the hydroalkylation step.
Both of the processes disclosed in U.S. Pat. No. 7,579,511 and WO2009/131769 rely on the use of a dehydrogenation catalyst comprising a Group VIII metal on a porous inorganic support such as aluminum oxide, silicon oxide, titanium oxide, zirconium oxide, activated carbon, and combinations thereof. However, in practice, such a dehydrogenation catalyst has only limited activity for the conversion of cyclohexane and/or methylcyclopentane and in some instances can undergo rapid aging. There is therefore a need for an improved catalyst for removing cyclohexane and methylcyclopentane from the benzene recycle compositions employed in benzene hydroalkylation processes. Conversion of cyclohexane is especially important since its boiling point is within 1° C. of that of benzene. Conversion of methylcyclopentane is also desired but less important than cyclohexane since there is a difference of nearly 9° C. in the boiling point of methylcyclopentane and benzene.
More recently, it was discovered that catalyst containing at least one dehydrogenation metal (e.g., platinum or palladium) and a Group 1 or Group 2 metal promoter (i.e., alkali metal or alkaline earth metals) can be used to dehydrogenate cyclohexane and/or methylcyclopentane. This process is described, for example, in PCT Application No. PCT/US2010/061041, which was filed on Dec. 17, 2010. However, dehydrogenation catalysts having further improved cyclohexane conversion and/or selectivity are needed.